US-backed Kurdish fighters in northern Syria say Turkish military vehicles are massing near the border ahead of an expected assault that could derail their fight against ISIS in the east of the country and further strain relations between Ankara and Washington, its Nato ally.

People's Protection Units (YPG) spokesman Nuri Mahmoud told The National that they are ready to intercept any attempt by Turkey to cross into the Kobani canton, east of the Euphrates River.

Turkey on Wednesday killed at least four Syrian Kurds, including at least two fighters, and wounded six others when it shelled the villages of Selim and Kor Ali west of Kobani, according to the Kurdish-run Hawar news agency.

The attack came one day after Turkish President Recep Tayyip Erdogan vowed to destroy Kurdish militias east of the Euphrates. "We have completed our preparations, plans, programmes regarding this issue,” he told members of his ruling Justice and Development Party in parliament.

Turkey considers the YPG militia an extension of the outlawed Kurdistan Workers' Party (PKK), which has led an insurgency in Turkey for more than three decades.

The US has worked closely with the YPG in the fight against ISIS in Syria, straining relations between Washington and Ankara.

The US State Department on Wednesday expressed “great concern” over Turkey’s shelling of Kurdish forces. “Unilateral military strikes into north-west Syria by any party, particularly as American personnel may be present or in the vicinity, are of great concern to us,” deputy spokesman Robert Palladino said. The US has been in touch with Turkey and a Kurdish-dominated Syrian militia to emphasise the need “to de-escalate the situation”, he said.

Mr Palladino said the US was fully committed to Turkey’s border security but at the same time expressed Washington’s commitment to the Kurdish-dominated Syrian forces. The campaign against ISIS “is not over and that fight remains difficult”, he said.

The threat of a Turkish assault comes at a time when hundreds of Kurdish fighters affiliated with the YPG and the Syrian Democratic Forces (SDF) have moved south from Kobani to Deir Ezzor province to battle ISIS in its last holdout near the Iraqi border. The Syrian Observatory for Human Rights said on Thursday that at least 100 more Kurdish fighters had arrived in the area less than 24 hours after Turkey shelled their positions in the north.

The SDF on Wednesday said it would temporarily halt its campaign against ISIS in protest over continued Turkish aggression in the Kobani region. The continuation of Turkish attacks, the group said, would further delay the offensive on the ISIS-held pocket of Hajin.

When asked whether Kurdish fighters in eastern Syria would be recalled to the north to boost defences, the YPG spokesman said there were enough fighters in the Kobani region to foil a Turkish attack, but fighters would be recalled if needed.

The Syrian rebel-affiliated Step News Agency reported on Thursday that Kurdish forces are moving armoured vehicles and heavy weaponry from eastern Deir Ezzor to Hassakeh, about 270 kilometres east of Kobani.

Meanwhile, west of the Euphrates River, joint US and Turkish forces were expected to begin joint patrols on Thursday as part of a road map for easing tensions between the Nato allies.

Sharfan Darwish, spokesman of the Manbij Military Council, told The Associated Press the patrols would be along the front lines between his group and Turkey-backed Syrian rebels.

Ankara and Washington agreed on a road map in June amid Turkish demands for the withdrawal of the YPG, which freed Manbij from ISIS in 2016.

The Americans and the Turks have been conducting independent patrols along the border, and joint patrols are seen as a way of preventing violence.

The Manbij Military Council that administers the town says the last Kurdish fighters left Manbij in July.

Turkey's state-run Anadolu Agency quoted Turkish Defense Minister Hulusi Akar as saying that despite promises that the YPG and other Kurdish militias "will be removed from Manbij, the terror organisations are digging trenches there like they did in Afrin". He was referring to a Kurdish enclave taken earlier this year by Turkish troops and Turkey-backed Syrian opposition fighters.

The terror organisation should know that they will be buried in the trenches when the time and place comes," Mr Akar said.